In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening fluid to the plate prior to inking. The dampening fluid prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas. Ink applied uniformly to the wetted printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
To circumvent the cumbersome photographic development, plate-mounting, and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers. Three-layer plates, for example, are made ready for press use by image-wise exposure to imaging (e.g., infrared or “IR”) radiation that causes ablation of all or part of the central layer, destroying the bonding to the overlying (typically polymer) layer in the exposed areas. This may involve complete ablation of the central layer or ablation of its upper region.
Subsequently, the de-anchored overlying layer and the central layer are removed (at least partially) by a post-imaging cleaning process—e.g., rubbing of the plate with or without a cleaning liquid—to reveal the third layer (typically an oleophilic polymer, such as polyester). If the central layer is metal (e.g., a very thin layer of titanium), the entire layer will be ablated, and the final printing member will feature unexposed polymer areas over metal and the underlying polymer layer (or layers). If the central layer is polymeric, partial (but de-anchoring) ablation of the layer can be tolerated under either of two conditions: the remainder of the layer is removed by cleaning, or the central layer is oleophilic (so persistence of some portion of that layer, even after cleaning, does not affect the plate's lithographic performance). The edges of the printing member may be pinned to a plate cylinder by metal clamps, which, due to their mechanical association with the press, are electrically grounded.
This type of plate structure has a tendency to undergo triboelectric charging during printing due to repetitive cycles of contact with and separation from the press form rollers (which, like the topmost polymer plate layer, are made of insulating material). Because the clamps provide a ground path, electrostatic charge accumulating on regions of the plate held by clamps dissipates or never develops. But unimaged islands within the plate, which have both polymer and metal layers, are electrically isolated from the clamps. As a result, the accumulated charge is trapped in these regions. The charge build-up is cumulative and therefore increases as a function of the speed of the printing process. (See, e.g., U.S. Pat. No. 6,055,906, the entire disclosure of which is hereby incorporated by reference.)
Under standard press operation conditions the static charge can build up rapidly and create high-voltage differences between the different areas of the printing member. The latter can lead to electrostatic discharge (“ESD”) events, when sudden and uncontrolled transfer of static charge occurs. The electrostatic energy is converted into heat that can cause severe damage to the fine features of an imaged plate, leading to unacceptable print-work.
In a waterless press (in which the printing member has, for example, a silicone topmost layer), the static charge accumulation and/or dissipation can be partially controlled by, for example, increasing the relative humidity of the room; using form rollers made of materials close to silicone in the triboelectric series; and/or using air-ionizing bars. These solutions are cumbersome and expensive, and frequently unrealistic in a commercial printing environment.